The present invention relates to a channel allocation method in a radio system comprising at least one base station communicating with terminals within its coverage area, some of the terminals using discontinuous transmission, and in which method air interface resources are allocated to different terminals with a combined time and code division multiple access method in such a way that a signal of one or more terminals is transmitted in each time slot.
One of the most essential problems in planning and implementing radio systems is the efficient use of frequency. As the volume of radio traffic is continuously on the increase, this problem has become more and more important. The more efficiently a system can utilize a given frequency band, the greater is the potential number of users of the system. When new radio systems are developed, the multiple access methods of air interface are a central field of research.
Thus the aim of multiple access methods is to use frequency, in other words air interface resources, as efficiently as possible. As the number of users increases, the most important factor that restricts the capacity is co-channel interference, and as a consequence the way how users are grouped into different channels becomes crucial to maximizing the capacity. Specifically in mobile communication systems the problem is on one hand the mobility of users, which gives rise to geographically varying needs of capacity, and on the other hand the environmental differences in view of the propagation of radio waves, and the multipath propagation, which cause slow and fast changes in signal strength. Thus a multiple access method must be applicable under the influence of random factors in a way which enables the division of channels between users advantageously in view of the whole system.
In frequency division multiple access (FDMA), users are separated from one another by frequency; a separate frequency band is allocated to the data signal of each user. In time division multiple access (TDMA) the frequency band is divided into successive time slots, and the data signal of each user is transmitted in a separate, repetitive time slot. A plurality of frequency bands like this can be in use, in which case the method is called a combined FDMA/TDMA. In a conventional TDMA system, when a transmitter is powerful, the same time slot cannot be used in adjacent cells due to co-channel interference, even though the aim is to limit the average transmission power by using discontinuous transmission. The problem has been solved by adopting a reuse pattern according to which a common frequency is not used in adjacent cells. However, this is inefficient as to the use of frequencies. Furthermore, the decrease in the average interference level caused by discontinuous transmission cannot be fully utilized to reduce the reuse configuration, since the reuse configuration is determined to a great extent by the peak value of the co-channel interference caused by one user.
Code division multiple access (CDMA) is a multiple access method based on a spread-spectrum technique. In conventional CDMA, a narrowband data signal of the user is modulated by a spreading code with a broader bandwidth than that of the data signal onto a relatively broad band. The spreading code comprises a plurality of bits. The bit speed of the spreading code is much higher than that of the data signal, and in distinction from the data bits and data symbols the spreading code bits are called chips. Each data symbol of the user is multiplied by all spreading code chips. Each user of the same cell has a separate spreading code. Several users transmit simultaneously on the same frequency band, and data signals are distinguished from one another at receivers on the basis of the spreading code. In CDMA systems the power adjustment of transmitters is assumed to function optimally in disturbance control. Users operating on different power levels are on the same frequency band and the function of power adjustment is critical to the system. Further, it is difficult to use great power differences in downlink transmission.
The method of the invention can be applied to radio systems specifically those employing a combined TDMA/CDMA method. Hence the air interface resources are divided in such a way that a channel consists of successive time slots transmitting the signals of one or more terminals, which signals are multiplied each by a separate spreading code, which may be one or more. FDMA can also be included in the method, in which case several different frequencies, each employing the combined TDMA/CDMA method, are in use. The combined TDMA/CDMA method is described in greater detail in Baier P. W, A Critical Review of CDMA, Proceedings of VTC96, pp. 6-10.
The aim of the present invention is to implement a method for multiple access and channel allocation by which the effects of random factors can be averaged efficiently, and which is easily adaptable in different operating and environmental circumstances.
This is achieved with a method set forth in the preamble, which is characterized in that terminal signals are grouped into different time slots on the basis of the connection characteristics of each terminal in such a way that signals of terminals employing discontinuous transmission are grouped into the same time slots and transmissions of terminals employing continuous transmission into time slots of their own.
The invention also relates to a channel allocation method in a radio system comprising at least one base station communicating with terminals within its coverage area, and which system comprises both circuit switched services and packet switched services, and in which method air interface resources are allocated to different terminals with a combined time and code multiple access method in such a way that the signal of one or more terminals is transmitted in each time slot. The channel allocation method according to the invention is characterized in that the terminal signals are grouped into different time slots on the basis of the connection characteristics of each terminal in such a way that the signals of terminals employing circuit switched services are grouped into the same time slots, and the transmissions of terminals employing packet switched services into time slots of their own.
The invention further relates to a radio system comprising at least one base station communicating with terminals within its coverage area, and employing a combined time and code division multiple access method in such a way that the signals of one or more terminals are transmitted in each time slot, and in which system some of the terminals are fixed and others mobile. The radio system according to the invention is characterized in that the terminal signals are grouped into different time slots on the basis of the connection characteristics of each terminal in such a way that the signals of fixed terminals are grouped into the same time slots and the transmissions of mobile terminals into time slots of their own.
Several advantages are achieved with the method of the invention. By grouping the terminals into different time slots in accordance with their power levels, co-channel interference can be decreased. As a consequence of this the reuse configuration can be reduced, which allows easier cellular planning and more efficient utilization of the frequency spectrum. The reduction in power level changes within one time slot also enables an improvement in the reliability of interference level measurements of the channel.
The method of the invention can thus be applied to TDMA/CDMA systems, and to systems which, in addition to the aforementioned, also comprise FDMA, that is to say a plurality of frequency channels. Hence in time slot allocation the availability of an adjacent channel can be considered depending on whether low or high power users are allocated to the time slot. Furthermore, the invention can be applied to both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) methods.